(The Guardian) – Barely a fifth of the way into their mission, the engineers monitoring the Scorpion’s progress conceded defeat. With a remote-controlled snip of its cable, the latest robot sent into the bowels of one of Fukushima Daiichi’s damaged reactors was cut loose, its progress stalled by lumps of fuel that overheated when the nuclear plant suffered a triple meltdown six years ago this week.

As the 60cm-long Toshiba robot, equipped with a pair of cameras and sensors to gauge radiation levels was left to its fate last month, the plant’s operator, Tokyo Electric Power (Tepco), attempted to play down the failure of yet another reconnaissance mission to determine the exact location and condition of the melted fuel.

Even though its mission had been aborted, the utility said, “valuable information was obtained which will help us determine the methods to eventually remove fuel debris”.

The Scorpion mishap, two hours into an exploration that was supposed to last 10 hours, underlined the scale and difficulty of decommissioning Fukushima Daiichi – an unprecedented undertaking one expert has described as “almost beyond comprehension”.

Cleaning up the plant, scene of the world’s worst nuclear disaster since Chernobyl after it was struck by a magnitude-9 earthquake and tsunami on the afternoon of 11 March 2011, is expected to take 30 to 40 years, at a cost Japan’s trade and industry ministry recently estimated at 21.5tr yen ($189bn).

The figure, which includes compensating tens of thousands of evacuees, is nearly double an estimate released three years ago. […]

Developing robots capable of penetrating the most dangerous parts of Fukushima Daiichi’s reactors – and spending enough time there to obtain crucial data – is proving a near-impossible challenge for Tepco. The Scorpion – so called because of its camera-mounted folding tail – “died” after stalling along a rail beneath the reactor pressure vessel, its path blocked by lumps of fuel and other debris.

The device, along with other robots, may also have been damaged by an unseen enemy: radiation. Before it was abandoned, its dosimeter indicated that radiation levels inside the No 2 containment vessel were at 250 sieverts an hour. In an earlier probe using a remote-controlled camera, radiation at about the same spot was as high as 650 sieverts an hour – enough to kill a human within a minute. [more]